The present invention relates to instruments for measuring minute concentrations of impurities in liquids, and more particularly to systems for detecting non-volatile residue concentrations in ultra pure water and aerosol generating components used in such systems.
Certain industries, most notably semiconductor fabrication, involve extremely high standards of cleanliness and purity. A semiconductor component may require washing with ultra pure water after each processing stage, to remove chemicals used in that stage. More generally, ultra pure water may be used to clean fixtures and other tools used to handle semiconductor wafers and other components. Any non-volatile residue present in the ultra pure water can remain on the surface of the component after the water has evaporated, possibly causing defects in the resulting semiconductor device. Thus, there is a need to monitor the ultra pure water used in such processes, to insure that the concentration of non-volatile residue remains at or below acceptable levels.
Systems have been developed and employed successfully to continuously monitor the quality of ultra pure water. U.S. Pat. No. 5,098,657 (Blackford et al.) discloses a system in which ultra pure water is provided at a constant flow rate to a nebulizer where the water is formed into droplets. The droplets are dried to provide non-volatile residue particles. The particles can be detected electrostatically or provided to a condensation particle counter (CPC, also known as a condensation nucleus counter) where the particles are “grown” into larger droplets and sensed optically. Droplets are grown with a fluid having a relatively low vapor mass diffusivity, e.g. butyl alcohol.
While such systems have enjoyed success, they also are subject to difficulties that limit their utility. One of these concerns is measuring the flow rate of the ultra pure water to the nebulizer. Such measurement is critical, because any change in the flow rate, e.g. due to a blockage in the water delivery system, seriously disrupts measurement of the residue concentration. The conventional approach is to position a rotometer just upstream of the nebulizer. Rotometers are not particularly well suited for measuring the extremely low flow rates involved, typically about 1 milliliter per minute. They are expensive, in part due to the need for ultra-clean materials such as Teflon to minimize residue contamination that would adversely affect concentration readings.
Another problem is the accumulation of waste water in the nebulizer. Conventional devices have employed sponges to absorb waste water, but this only postpones the eventual need to remove the waste water.
Another difficulty concerns the sapphire orifice plate typically used to control the flow rate of water into the nebulizer. A forty micron diameter orifice through the plate limits the flow and admits water into the nebulizer. The pressure drop across the orifice plate is sufficient to cause gasses dissolved in the water to accumulate on the back side of the orifice plate and form bubbles. Downstream of the nebulizer, the bubbles eventually break free and tend to disrupt residue concentration measurements.
As to the condensation particle counter, a concern relates to the use of butyl alcohol or similar fluids with low vapor mass diffusivity for growing the residue particles into droplets. Such liquids tend to be flammable, toxic, and produce noxious odors that frequently require vapor exhaust systems to be located near the measuring device. Frequently the liquids are subject to health and environmental regulations that restrict their use in indoor environments. In addition, the liquids require equipment for supplying, collecting and draining the liquid involved.
Another persistent problem is the relatively long time elapsed between a change in the concentration of non-volatile residue in the ultra pure water, and the detection of the change. This raises the risk that contaminated water may be used in several process stages before the condition is realized.
Accordingly, the present invention has several aspects directed to one or more of the following objects:                to provide an aerosol generating device, e.g. a nebulizer, with a reliable means for measuring an input flow rate of the liquid supplied to the device without contacting or otherwise interfering with the liquid used to generate the aerosol droplets;        to provide an aerosol generating device particularly well-suited for digital measurement of the flow rate of the liquid into and through the device;        to provide a system and process for measuring concentrations of non-volatile residue in test liquids, adapted to facilitate the use of the test liquid as the condensation medium for droplet growth onto previously dried particles for optical detection; and        to provide a non-volatile residue measuring system with improved response times for alerting users to changes in non-volatile concentrations, liquid flow rates and other key parameters.        